Mail sent to receiver but bounces

Whenever I sent a mail to some specific email of our domain, they get the mail, but I also receive a bounced message. Here is the bounce mail: [QUOTE]This is the mail system at host relay.mailchannels.net. I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to postmaster. If you do so, please include this problem report. You can delete your own text from the attached returned message. The mail system : host gmail-smtp-in.l.google.com[74.125.142.26] said: 550-5.2.1 The email account that you tried to reach is disabled. Learn more at 550 5.2.1